In the oil and gas industry, hydraulic fracturing involves injecting a specially blended fracturing fluid through a wellbore and into a formation under sufficiently high pressure to create fractures, thereby providing channels through which formation fluids such as oil, gas or water, can flow into the wellbore and thereafter be withdrawn. Fracturing fluids are designed to enable the initiation or extension of a fracture and the simultaneous transport of suspended proppant (for example, naturally-occurring sand grains, resin-coated sand, sintered bauxite, glass beads, ultra lightweight polymer beads and the like) into the fracture to keep the fracture open when the pressure is released. The performance and the ability of a fracturing fluid to carry proppant are largely dependent upon its viscous properties. Additional desirable characteristics of a fracturing fluid include the ability to be broken and cleaned out of the fracture following the treatment, good fluid-loss control and low friction pressures during application. Common fracturing fluids are based upon either aqueous or hydrocarbon systems, although aqueous fluids (for example, those based on water-soluble polymers, guar gums and guar derivatives) are generally more popular due to lower costs. While it is possible to increase the viscosities of guar-based fluids by elevating the concentration, a more economical approach involves cross-linking the polymers by applying cross-linking agents.
Polymer-free, water-based fracturing fluids may be prepared using surfactants. Compared to a common gel prepared from guar derivatives, a surfactant-based fracturing fluid minimizes the amount of residue remaining in the formation after the treatment. Depending on the grade of the selected guar polymer, the residue can be significant and impede the success of the fracturing procedure. The residue typically includes not only breakdown products resulting from the enzymatic or oxidative decomposition of the polymer structure following the treatment, but also contamination arising during processing of the guar. While modified guars usually contain fewer contaminants due to additional purification, such contaminants cannot be eliminated completely and economically.
Surfactant-based systems are purely synthetic and thus not dependent on the weather or economically-related changes in the harvest of the raw material (for example, guar beans) which may influence availability on the world market. Surfactant-based systems form stable foams when applied under energized conditions. Compared to guar-based fluids, it is possible to obtain the desired fluid property without the addition of a foaming agent. Energized fluids require less base fluid, allowing for application in water-sensitive formations and decreasing the amount of chemical additives needed for the treatment. The reduced amount of fluid that needs to be flowed-back can be of importance in places where the disposal of waste fluid comprises a significant cost factor.
Surfactant-based fracturing systems are well known and valued for their ability to withstand high shear applications. Preferred surfactants can have a range of ionic character; anionic, non-ionic, cationic and zwitterionic species have all been used successfully. However, some cationic surfactants are toxic and are not readily biodegradable.
Viscous fracturing fluids must also be able to be ‘broken’, by disruption of the structure that causes the increase in viscosity of the fluid in first place. Depending on the composition of the fluid, this disruption can be achieved either by physical or chemical means. If accomplished by an additive, the additional chemical should have no, or only a minimal effect on the gel performance during the actual treatment, but should react rapidly once the treatment is finalized. It is important that the method allows for a certain degree of control over the time involved in the decrease of the gel strength, whereby the formation temperature and pressure may play a vital role.
Typical breaker additives used in combination with guar based fracturing fluids are oxidizers and enzymes. Oxidizing chemicals like ammonium, potassium or sodium salts of peroxydisulfate cause the radical decomposition of the carbohydrate polymers, reducing their molecular weight and therefore their viscosifying ability. Certain amine based additives are available that can enhance the reactivity of the breakers.
Enzymatic breakers provide a less aggressive way to degrade carbohydrate based polymers. Common enzymes used in the oilfield are hemicellulases. Their application is limited to a smaller pH range (3.5 to 8) and lower temperatures compared to oxidizing breakers.
Surfactant-based fracturing fluids can be applied without breaker, depending solely on either the dilution of the network with formation water or the disruption of the micelles by contact with a sufficient amount of hydrocarbon, to reduce the viscosity of the fluid. However, this approach has the disadvantage that there is no means of control over the duration of the ‘break’. Furthermore, it may be less economical due to the increased shut-in time of the well.
Although various fracturing fluids are presently used, there remains a need for fracturing fluid systems which mitigate disadvantages of the prior art formulations.